(1) Field of the Invention
This invention relates to an apparatus and method for chemical-mechanical polishing (CMP) a semiconductor substrate and more particularly to an apparatus and method for conditioning the polishing pad in order to control the polish removal rate and prolong the life of the polishing pad.
(2) Description of Related Art
Chemical-mechanical polishing (CMP) has been developed for providing smooth topographies on surfaces deposited on semiconductor substrates. Rough topography results when metal conductor lines are formed over a substrate containing device circuitry. The metal conductor lines serve to interconnect discrete devices, and thus form integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin layers of insulating material and holes formed through the insulating layers provide electrical access between successive conductive interconnection layers. In such wiring processes, it is desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces. CMP can, also, be used to remove different layers of material from the surface of a semiconductor substrate. For example, following via hole formation in an insulating material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal studs.
Briefly, the CMP processes involve holding and rotating a thin, flat substrate of the semiconductor material against a wetted polishing surface under controlled chemical, pressure and temperature conditions. A chemical slurry containing a polishing agent, such as alumina or silica, is used as the abrasive material. Additionally, the chemical slurry contains selected chemicals which etch various surfaces of the substrate during processing. The combination of mechanical and chemical removal of material during polishing results in superior planarization of the polished surface.
The wetted polishing surface comprises a porous pad material, such as blown polyurethane, saturated with the polishing slurry. Mounting of the polishing pad to the polishing apparatus is a labor intensive operation and the mounting process, also, interrupts use of the polishing apparatus. The initial cost of the polishing pad, labor cost for mounting the pad to the polishing apparatus, and reduced throughput of the apparatus due to the polishing apparatus down-time while mounting the polishing pad add to the cost of polished product. Therefore, it is desirable to prolong the life of a polishing pad. A principal factor in polishing pad degradation is a phenomenon referred to as "glazing", in which, during use, abrasive particles from the polishing slurry and polished by-product become embedded and packed into the pores of the polishing pad. The result of "glazing" is a reduction of polish removal rate and underpolishing of product until a correction is made. FIG. 1 shows polish pad removal rate versus accumulated polishing time on a polishing pad. In this example, the polishing pad removal rate is significantly degraded after about 250 min. of accumulated polishing time. A technique used to overcome "glazing" is to periodically condition the polishing pad to rid the pad of embedded abrasive particles and polished by-product. State-of-the-art conditioning techniques include liquid rinsing, air blowing the polishing pad surface, and grinding of the polishing pad surface to expose a fresh surface. The grinding technique is typically accomplished by using a rotating diamond wheel to remove a portion of the pad surface. FIG. 2 shows polish pad removal rate versus accumulated polishing time, degradation of the polish pad removal rate over time, and restoration of the polish pad removal rate following pad conditioning using grinding with a diamond wheel to remove a layer of the polish pad surface. In this example, pad conditioning at about 350 min. accumulated polish time, restores the degraded polish pad removal rate to the "fresh pad" removal rate. It is important to know when pad conditioning is necessary and when the pad conditioning operation is effective. Unnecessary cost is added to the polishing process if pad conditioning is done before "glazing" has reduced the polish removal rate. It is, also, important to know when the pad conditioning operation is effective because under-conditioning will not restore the polish removal rate to the "fresh pad" polish removal rate and over-conditioning will excessively consume the polishing pad and will thereby decrease the polish pad life.
Polish pad life is a subject of concern in current CMP technology, as shown in the U.S. Pat. Nos. 5,310,455 and 5,232,875. U.S. Pat. No. 5,310,455 entitled "Techniques For Assembling Polishing Pads For Chemi-Mechanical Polishing of Silicon Wafers" granted May 10, 1994 to Nicholas F. Pasch et al describes a method of mounting polishing pads to a polishing apparatus, wherein the polishing slurry solution is diverted away from the adhesive interface between pads, thereby prolonging the life of the polishing pad by reducing catastrophic delamination of the polishing pad from the polishing apparatus. U.S. Pat. No. 5,232,875 entitled "Method and Apparatus For Improving Planarity of Chemical-Mechanical Planarization Operations" granted Aug. 3, 1993 to Mark E. Tuttle et al describes an improved polishing pad having a porous surface and perforations which extend from a lower surface thereof to an upper surface thereof. The perforations effect efficient distribution of the polishing slurry and prolong the life of the polishing pad.
The present invention is directed to a novel method and apparatus for dynamic control of polishing pad conditioning processes in order to prolong the life of the polishing pad, maintain the non-degraded polish removal rate for the polishing pad, and improve the product throughput of the polishing apparatus.